The present invention relates to an optical wavelength-division multiplex transmission system for transmitting information using a plurality of optical signals having different wavelengths.
There has been an increasing demand for an optical fiber transmission system has been increasing for transmission of a wide band signal such as video signals, and/or a large capacity signal through transoceanic links. In such an optical fiber transmission system, however, a distance between an electro-optical (E/O) converter and an opto-electrical (O/E) converter is limited due to a transmission loss in the optical fiber. In order to solve this problem, an optical transmission system has been proposed by J. C. Simon in an article entitled "Semiconductor Laser Amplifier for Single Mode Optical Filter", Journal of Optical Communications, vol. 2, pp. 51-61, April 1983 (Reference 1). In the article, an optical amplifier is used to expand the interval distance by amplifying a single optical signal as it is. Since the optical amplifier produces spontaneous emission noise during amplification of the optical signal, Simon uses an optical filter to remove the spontaneous emission noise.
In addition, according to the fact that a single optical fiber can transmit a plurality of optical signals having different wavelengths, an optical wavelength-division multiplex (WDM) transmission system has been studied in which optical signals supplied from individual E/O conventers are combined into the single optical fiber for transmission of much information.
The WDM transmission system is also restricted in an interval distance between the E/O and O/E converters. Therefore, it may be also desirable in the WDM system to provide a laser amplifier to increase the light power, as used in the single wavelength system. However, it has been never proposed to use such a laser amplifier in an optical WDM transmission system.
Further, the WDM transmission system has the problem that the optical amplifier used therein tends to be more saturated by the spontaneous emission noise and the optical signals transmitted to the amplifier than the optical amplifier used in the single wavelength system. Accordingly, unless the noise is suitably suppressed, the optical signal in the WDM transmission system can not be amplified satisfactorily. This problem has never been considered.